CVE-2021-31988Network attack vector

SMTP header injection via unvalidated subject parameter

Published Oct 5, 2021 · Updated Nov 8, 2024

CRLF injection in Axis Communications AB AXIS OS 5.51 or later allows attackers to add arbitrary SMTP headers through a crafted subject. The /axis-cgi/smtptest.cgi endpoint places CRLF characters from the HTTP subject parameter into the generated test message instead of rejecting them. Exploitation requires network and administrator access to the device, or a logged-in administrator induced to visit a crafted page, and changes the headers of an SMTP test email.

CVSS severity8.8
High
EPSS probability0.95%
Next 30 days · Sep 17, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

CRLF injection in Axis Communications AB AXIS OS 5.51 or later allows attackers to add arbitrary SMTP headers through a crafted subject. The /axis-cgi/smtptest.cgi endpoint places CRLF characters from the HTTP subject parameter into the generated test message instead of rejecting them. Exploitation requires network and administrator access to the device, or a logged-in administrator induced to visit a crafted page, and changes the headers of an SMTP test email.

The record

CVE
CVE-2021-31988
Published
Oct 5, 2021
Updated
Nov 8, 2024
Vendor
Axis Communications AB
Product
AXIS OS
Classifications
CWE-1286, CWE-74
Attack vector
network
Privileges
Unavailable

Timeline

How it unfolded

  1. Oct 5, 2021CVE publishedPublication date reported by the CVE source.
  2. Nov 8, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=AXIS OS 5.51 or later

What conditions does exploitation require?

Attack vectornetwork
Required privilegesUnavailable in this record.

What is affected?

Axis Communications AB · AXIS OSversion=AXIS OS 5.51 or later

Published CVSS scores

6.8NIST National Vulnerability DatabaseAV:N/AC:M/Au:N/C:P/I:P/A:P
8.8NIST National Vulnerability DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 11, 2026Sep 17, 2026
Latest reporting daySep 17, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
PrivilegesUnavailable
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-1286Improper Validation of Syntactic Correctness of Input
CWE-74Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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